1. Field of the Invention
The present invention relates to a hydraulic control system for controlling hydraulic driven actuators and a construction machine using the hydraulic control system.
2. Description of the Related Art
In a hydraulic excavator, when a swing motion is to be stopped, a control valve for a swing motor is returned to its neutral position to cut off a supply of pressure oil from a hydraulic pump to the swing motor. However, since the swing motor continues to rotate for a certain time due to a large inertia of an upper rotating body, there occur cavitations.
Further, if an arm cylinder is extended in the air and an arm pulling operation is performed while an engine speed is set in a low idling condition, the arm pulling operation is accelerated under the action of the arm weight and hence pressure oil fed to the head side of the arm cylinder becomes short. In such a case, there also occur cavitations.
The following methods have been proposed as means for decreasing a loss of energy while preventing cavitations.    (a) A pilot pressure of a traveling/rotating operation pilot valve is detected through a shuttle valve and is conducted to a back pressure proof valve, while a back pressure is developed in the back pressure valve in traveling and rotating operation to prevent an occurrence of cavitations, while in other operations the back pressure is not developed to decrease the loss of energy (see, for example, Japanese Patent Laid-Open No. Hei 7-180190).    (b) A drive-side pressure of a hydraulic motor or a hydraulic pump pressure is taken out as a pilot pressure, and a variable throttle valve which utilizes the said pilot pressure to switch a back pressure to a low or high pressure is provided in a back pressure circuit (see, for example, Japanese Patent Laid-Open No. Hei 9-317879).    (c) A bypass valve is disposed in a bypass which is formed in parallel with a back pressure check valve and is closed only when a hydraulic actuator is stopped, causing a back pressure to be developed by the back pressure check valve (see, for example, Japanese Patent Laid-Open No. 2002-89505).
The cavitation preventing circuit in the above (a) and (b) is configured so as to make prevention of cavitations and decrease of energy loss compatible with each other by switching the condition of back pressure which is for preventing the cavitations of a swing motor (rotating motor) and a travel motor. However, no consideration is given, for example, to hydraulic cylinders for actuating a front attachment and it is impossible to decrease the loss of energy throughout the whole hydraulic control circuit.
In the case of an actuator wherein an incoming flow rate and an outgoing flow rate are equal as in a hydraulic motor, the outgoing flow rate does not exceed the incoming flow rate from a hydraulic pump, but in case of a hydraulic cylinder, cavitations are apt to occur when the cylinder is extended in its extending direction due to a difference in sectional area between an oil chamber formed on the head side and an oil chamber on the loss side. Conversely, however, in case of operating the hydraulic cylinder in its retracting direction, the outgoing-side flow rate is large and a back pressure is developed due to the resulting pressure loss of an actuator pipe. Therefore, cavitations are difficult to occur. In the conventional circuit for the prevention of cavitations, return oil flows through a back pressure circuit also in the cylinder retracting direction, so that the loss of energy is large. It is necessary to decrease such energy loss generated in the hydraulic cylinder.
In the cavitation preventing circuit in the above (c), a negative control pressure is utilized for closing the bypass valve, and only when all the actuators (hydraulic motor and hydraulic cylinders) are stopped, the bypass valve is closed with a negative control pressure and a back pressure is generated by the back pressure check valve. On the other hand, while the actuators are not stopped, the bypass valve is opened, allowing the back pressure check valve to function as a bypass to prevent the loss of energy. According to this configuration, the loss of energy cannot be decreased in the case where the hydraulic cylinders are operated each independently.